I am currently building a network consisting of several >10km links, so we are using ZX GBICs. However, some of the links are under 30km, so according to http://www.cisco.com/en/US/products/hw/modules/ps872/products_data_sheet09186a008014cb5e.html, we need to attenuate the signal in order not to overload the Rx side.
I am currently working on a connection that is about 22km. The fiber path is already available and I've had it measured to know the fiber attenuation, which is 8.5 dB.
According to the mentionned document, the Tx power of the WS-G5487 / 1000BASE-ZX is "minimum 0dBm, maximum 5dBm"; the Rx should be "minimum -23, maximum -3". For what I have understood is that the goal to aim for is to be right in the middle, being -13dBm.
So, I could have:
0dBm - 8.5dB - 5dB = -13.5dBm
5dBm - 8.5db - 10dB = -13.5dBm
So depending on the output power of the GBIC, I should either put in 5dB or 10dB attenuation.
My question is: how does the GBIC determine it's output power? Does it increase output power when it sees a weak signal? Or has every GBIC a fixed output power somewhere between 0 and 5dB? How do I know which Tx power value I should use in my calculations?
(I now solve it by measuring light on the Rx at the "far" end side before connecting attenuator + fiber to the switch, and I will continue to do so, but I'd really like to know the 'why' behind it, so I can better understand the process - and know what I can expect when troubleshooting)
A ZX GBIC optical output power is fixed by the manufacturer, but may fall in a range from 0dBm to +5dBm. This has to do with manufacturing tolerances. When determining if you need an attenuator, pick the worst case (i.e. +5dBm) or do a measurement on the actual GBIC at hand.
The maximum Rx power is -3dBm. So any fiber link over 8dB is fine. But as you said: it's common practive to engineer your link so as to hit the receiver at a power of -13dBm.
You have a 8.5dB fiber link. For safety reasons, always assume that your GBIC emits maximum power (+5dBm) but make sure that, if it is only 0dBm, the link is still within receiver tolerances.
Also please note that a laser degrades a bit over its operating lifetime, but should always provide more than the minimum of 0dBm (if not: replace it).
For the link you mention, Both a 5dB or 10dB attenuator will provide a working link. Personally, I'd put in 5dB, as the laser may degrade a bit over time.
hope this helps,